pgsql: Fix to_date() and to_timestamp() to handle year masks of length

Fix to_date() and to_timestamp() to handle year masks of length < 4 so
they wrap toward year 2020, rather than the inconsistent behavior we had
before.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/029dfdf1157b6d837a7b7211cd35b00c6bcd767c

Modified Files
--------------
doc/src/sgml/func.sgml             |    9 +++++
src/backend/utils/adt/formatting.c |   58 ++++++++++++++++++++----------------
2 files changed, 41 insertions(+), 26 deletions(-)